1. Field of the Invention
The present invention generally relates to heat sinks for semiconductor devices, and more particularly, to a surface complemental heat dissipation heat sink.
2. Description of the Related Art
Heat is the flow of energy due to the existence of a temperature differential between two geographically separated points. One of the basic tenants in designing semiconductor devices is to ensure the devices operate at safe temperature levels.
Poorly designed heat sinks result in large thermal resistances, high semiconductor temperatures, poor electrical performance, and premature failure. Any design, therefore, must provide for adequate heat removal.
It is common practice to view the thermal flow path as a series of resistances (R) between power dissipating chips and the heat sink. Thermal resistance is somewhat analogous to electrical resistance in that heat flow is directly proportional to the temperature drop in the direction of the heat flow path. In flowing from the chip to the package surface or case, the heat encounters a series or resistances associated with individual layers of materials such as silicon, solder, copper, alumina, and epoxy, as well as the contact resistances that occur at the interfaces between pairs of materials. The amount of heat generated can be quantified via simple calculations involving the relevant thermal parameters. These are power dissipation (Q), in watts (W), the temperature difference between endpoints (T), in .degree.C., and thermal resistance (R), in .degree.C./W, where Q=T/R.
Advances in microprocessor technology have resulted in present day chips with greatly increased device densities that are operating at higher and higher clock speeds. For example, in a prototype 64 Mb DRAM chip, up to 140 million transistors are placed on a 1 cm by 2 cm silicon chip, with a feature size of 0.3-0.4 .mu.m. Based on current trends, thermal management of multi-chip modules can be expected to require removal of as much as 25-35 W/cm.sup.2.
Obviously, without adequate heat removal and thermal management, the transistor device will experience a destructive rise in junction temperature above the maximum allowable level. It is critical, therefore, to provide an effective heat dissipation system for the microprocessor to ensure system reliability.
In most systems, heat dissipation is accomplished through forced and natural convection, and typically a combination of the two. Convection is the transfer of heat to or from a surface by a moving fluid, such as air. Fluid motion may be produced by a fan (i.e., forced convection) or may result from buoyancy effects due to the presence of temperature gradients within the fluid (i.e., natural convection). Forced convection may include a system fan or a fan mounted directly on top of the microprocessor. Aluminum heat sinks directly attached on the microprocessor are a widely used method of natural convection.
There are drawbacks, however, to both methods of heat dissipation. In forced convection, for example, while most fans are generally effective in dissipating heat, they do require an isolated power supply and must be designed to be noise-free. Additional reliability risks and manufacturing and maintenance costs are also introduced. Furthermore, many military and telecommunications applications must meet design specifications which require equipment or systems that can be run without fans for a certain period of time. Finally, when using a cooling fan one must assess the nature of the moving air in the region near the heat sink. Not only is the air velocity an important factor, but one must take into account whether the air motion is laminar or turbulent.
Regarding natural convection, we know that the effectiveness of a natural heat sink is directly proportional to its surface area. In a microelectronic system, however, the surface area available for a heat sink is usually limited by the allowed area and height on a printed circuit board (PCB). In other words, the available height of the heat sink is limited to ensure it does not extend into a neighboring PCB. Nor should the surface area of the heat sink be so large as to cover other components or test points on the particular PCB.
Thus, a need exists for a heat dissipation device that increases the natural heat dissipation capacity of conventional microelectronic systems while working within the space constraints of the systems.